The present invention relates in general to flow meters and in particular to an electromagnetic pick-up arrangement in flow meters particularly in flow meters of small nominal width. The flow meter is of the type which includes a measuring chamber, a measuring wheel supported for rotation in the chamber and including a magnetic device having pole surfaces directed against an end wall of the measuring chamber. The end wall is of a magnetically non-conductive material and is formed with a well or space in which a Wiegand sensor is fixedly arranged in such a manner that during each rotation of the measuring wheel electric pulses are generated in the sensor whereby the number of pulses is a measure of flow of a medium streaming through the measuring chamber.
A pulse pick-up of this kind is described for example in the German publication DE-OS No. 3,046,804 which discloses a Wiegand sensor for impeller type counters in flow meters with mechanism and indicator operating in air. In this prior art arrangement, a ring magnet is embodied in the hub of the impeller to rotate therewith and to act on a single Wiegand wire which is surrounded by a sensing coil and oriented transversely to the axis of rotation of the impeller and closing the latter midway of its length. The Weigand wire is arranged outside a separating wall. The disadvantage of this known arrangement is the limited number of generated pulses inasmuch during each rotation of the impeller only a single pulse is generated and consequently resolution of the measured value low. Moreover this relatively large ring magnet cannot be installed in flow meters of very small nominal width because in such flow meters the measuring gears may be of smaller size than the required dimensions of the ring magnet. In this case, the ring magnet must be driven as a separate rotating member by the measuring wheel and consequently the rotating mass would be substantially increased.
In the German magazine "Electronic", 1980, copy 7, page 45, the application of Wiegand wires as a sensing module in flow meters has been devised. In this article an arrangement is described in which a number of Wiegand wires is arranged axis parallel on the jacket of a sensing drum rotating together with the measuring wheel. The sensing coil of the module together with magnets are disposed in a separate structural unit which must be mounted on the outer surface of a stationary housing part of the flow meter and arranged on the sensing drum. The disadvantage of this solution is the fact that the sensing drum which is necessary for supporting and embodying the Wiegand wires rotates partially in the work space of the counter and requires additional installation space. Furthermore, the sensing drum introduces a relatively large additional rotating mass exerting breaking action on the measuring wheel. Moreover, in this known solution the sudden polarity change in the Wiegand wires must be transmitted over a relatively large distance through the housing jacket of the flow meter to the outer fixedly mounted sensing coil and consequently only relatively weak pulses are produced. Since the flow meter counters are usually made of metal an additional damping of transmitted pulses occurs. The prior art pulse pick-up arrangement is unsuitable for installation in flow meters of small nominal diameters inasmuch the sensing drum carrying the Wiegand wires is too large in relation to the diameter of the measuring wheel and therefore it would make a compact construction of the flow meter impossible.